Process for production of a borohydride compound

ABSTRACT

A process for production of a borohydride compound. The process comprises the steps of: (a) combining a boron-containing salt, at least one of a metal and its hydride; wherein the metal is Be, Mg, Ca, Sr, Ba, Al, Ga, Si or a transition metal; and a solvent in which the borohydride compound is soluble; (b) grinding a mixture formed in step (a) to form the borohydride compound; and (c) separating a solution comprising the borohydride compound.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This is a divisional application of prior U.S. application Ser. No.11/098,734 filed on Apr. 4, 2005 now U.S. Pat. No. 7,297,316.

BACKGROUND

This invention relates generally to a process for production of aborohydride compound from boron-containing salts and metals or metalhydrides.

Production of sodium borohydride from sodium metaborate and alkali metalhydrides or alkaline earth hydrides or aluminum hydride is described inU.S. Pat. No. 3,140,150. An equation describing the reaction formagnesium hydride is as follows:NaBO₂+2MgH₂→NaBH₄+2MgOIt is also known in the prior art to react sodium borate salts, aluminumand hydrogen to produce sodium borohydride, as follows:3NaBO₂+4Al+6H₂→3NaBH₄+2Al₂O₃However, elevated temperatures and pressures typically are required inthe prior art processes. Moreover, the number of disclosed metals andmetal hydrides is limited.

The problem addressed by this invention is to find an efficient andeconomical process for production of a borohydride compound fromboron-containing salts and metals or metal hydrides.

STATEMENT OF INVENTION

The present invention is directed to a process for production of aborohydride compound. The process comprises the steps of: (a) combininga boron-containing salt, at least one of a metal and its hydride;wherein the metal is Be, Mg, Ca, Sr, Ba, Al, Ga, Si or a transitionmetal; and a solvent in which the borohydride compound is soluble; (b)grinding a mixture formed in step (a) to form the borohydride compound;and (c) separating a solution comprising the borohydride compound.

The present invention is further directed to a process for production ofa borohydride compound. The process comprises the steps of: (a)combining a boron-containing salt, and at least one of a metal and itshydride; wherein the metal is Al, Si or a transition metal; and (b)grinding a mixture formed in step (a) to form the borohydride compound.

DETAILED DESCRIPTION

All percentages are weight percentages based on the entire compositiondescribed, unless specified otherwise. A “transition metal” is anyelement in groups 3 to 12 of the IUPAC periodic table, i.e., theelements having atomic numbers 21-30, 39-48, 57-80 and 89-103. A“boron-containing salt” is an acid or salt containing a complex anion ofboron, preferably a complex anion containing only boron and oxygen. Mostpreferably, a boron-containing salt is an acid or salt containing a B₄O₇⁻² or BO₂ ⁻¹ ion, preferably the sodium salt. Preferably, theborohydride compound is sodium, potassium or calcium borohydride; mostpreferably sodium borohydride; and the boron-containing salt is a sodiumsalt. If a sodium salt of a boron compound having unequal molar amountsof sodium and boron, e.g., Na₂B₄O₇ is used as the boron-containing salt,preferably sodium hydroxide is added to provide the preferred Na:B molarratio of 1:1.

In those embodiments using a solvent, suitable solvents are those inwhich the borohydride compound is soluble and which are relativelyunreactive with borohydride, and with the metal and/or metal hydrideused. A solvent in which the borohydride compound is soluble is one inwhich the borohydride compound is soluble at least at the level of 2%,preferably, at least 5%. Preferred solvents include liquid ammonia,alkyl amines, heterocyclic amines, alkanolamines, alkylene diamines,glycol ethers, amide solvents (e.g., heterocyclic amides and aliphaticamides), dimethyl sulfoxide and combinations thereof. Preferably, thesolvent is substantially free of water, e.g., it has a water contentless than 0.5%, more preferably less than 0.2%. Especially preferredsolvents include ammonia, C₁-C₄ alkyl amines, pyridine,1-methyl-2-pyrrolidone, 2-aminoethanol, ethylene diamine, ethyleneglycol dimethyl ether, diethylene glycol dimethyl ether, triethyleneglycol dimethyl ether, tetraethylene glycol dimethyl ether,dimethylformamide, dimethylacetamide, dimethylsulfoxide and combinationsthereof. Preferably, the metal oxide produced in the reaction issubstantially insoluble in the solvent. Preferably, the solubility ofthe metal oxide is less than 0.1%. Use of a solvent also allows thereaction to be run more easily as a continuous process. Moreover, thesolvent facilitates heat transfer, thereby minimizing hot spots andallowing better temperature control. Recycle of the solvent is possibleto improve process economics. In another embodiment of the invention, amineral oil is used as the solvent to allow higher reactiontemperatures. Separation of the borohydride compound from the oil may beaccomplished via an extraction process after the oil is removed from thereactor.

The method of this invention uses at least one of a metal and itshydride, i.e., one or more metals may be present, one or more metalhydrides may be present, or a combination of metals and metal hydridesmay be present. The term “metal hydride” refers only to a simple metalhydride which is a compound of a single metal and hydrogen, and not tocomplex hydrides, e.g., lithium aluminum hydride. The metals and metalhydrides are selected from Be, Mg, Ca, Sr, Ba, Al, Ga, Si and thetransition metals. Preferably, the metals and metal hydrides areselected from Be, Mg, Ca, Sc, Y, La, Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Al and Si. More preferably, the metals and metal hydrides areselected from Mg, Ca, La, Ti, Zr, Zn, Al and Si. In one preferredembodiment, the metals and metal hydrides are selected from Mg, Ca, Zn,Al and Si, preferably as the metal rather than the metal hydride. Inanother embodiment of the invention, elements are chosen to maximizereaction efficiency. For example, the general reactions of sodiummetaborate with metals, M, or their hydrides, M_(x)H_(y), are as shownbelow:NaBO₂+4x/y M+2H₂→NaBH₄+4/y M_(x)O_(y/2)NaBO₂+4/y M_(x)H_(y)→NaBH₄+4/y M_(x)O_(y/2)Reaction efficiency would be maximized by selecting metals for which theratio x/y is minimized; for example, the metals and/or metal hydrides ofLa, Y, Sc, Ti, Zr, Al and Si.

Grinding of the reactants will accelerate the reaction, and may beachieved using any method which applies energy to solid particles toinduce a mechanochemical reaction, especially any method which reducessolids to the micron size range, preferably the sub-micron size range,and continually exposes fresh surfaces for reaction, e.g., impact, jetor attrition milling. Preferred methods include ball milling, vibratory(including ultrasonic) milling, air classifying milling, universal/pinmilling, jet (including spiral and fluidized jet) milling, rotormilling, pearl milling. Especially preferred methods are planetary ballmilling, centrifugal ball milling, and similar types of high kineticenergy rotary ball milling. Preferably, milling is performed in either ahydrogen atmosphere, or an inert atmosphere, e.g., nitrogen. In anembodiment in which a solvent is used, grinding of the reactants may beachieved using any method suitable for grinding a slurry.

Another method to accelerate the reaction is to use radiation techniquesalone or in combination with reactive milling. For example, microwaveirradiation can direct energy at specific reaction surfaces to providerapid heating and deep energy penetration of the reactants. Microwaveabsorbers such as metal powders, which could be used as milling media,and dipolar organic liquids may also be added to the reaction system topromote the reaction. The advantage of these techniques is that highreaction rates may occur at considerably lower processing temperaturethan could be obtained with resistive heating thermal techniques.

Without being bound by theory, it is believed that methods allowing useof a lower reaction temperature are beneficial, as the reactionequilibrium becomes less favorable at higher temperatures. Preferably,the reaction temperature is less than 250° C., and more preferably lessthan 150° C., when the grinding is carried out without a solvent. When asolvent is used, the preferred reaction temperature is below the boilingpoint of the solvent at the pressure within the grinding equipment.Preferably, the pressure is in the range from 100 kPa to 7000 kPa, morepreferably from 100 kPa to 2000 kPa.

Materials that catalyze surface hydride formation from gas phasehydrogen can be used to further hydriding kinetics. Examples of suitablecatalysts include powders of the transition metals, and their oxides,preferably La, Sc, Ti, V, Cr, Mn, Fe, Ni, Pd, Pt and Cu; oxides ofsilicon and aluminum, preferably alumina and silica; and AB₂, AB₅, AB,and A₂B types of alloys, wherein A and B are transition metals, such asFeTi and LaNi₅. A comprehensive list of hydriding alloys is given at theSandia National Laboratory website at hydpark.ca.sandia.gov/.

In one embodiment of the invention in which the boron-containing salt iscombined with a metal, hydrogen gas is necessary, as shown in theequations provided above. In this embodiment, the pressure of hydrogenpreferably is from 100 kPa to 7000 kPa, more preferably from 100 kPa to2000 kPa.

After the reaction has proceeded substantially to completion, preferablythe borohydride product is separated from the metal oxide byproducts. Inone embodiment in which a solvent is used, the solvent is separated fromthe insoluble metal oxide product and any grinding medium, which alsowould be insoluble. The borohydride compound can then be separated fromthe solvent by conventional methods. For example, the borohydridecompound-rich solvent can removed by filtering or using any otherconventional solid-liquid separation device such as a centrifuge. Theinsoluble solid metal oxide is collected and dried. High purityborohydride compound can be recovered from the solvent phase byevaporating the solvent or by lowering temperature to crystallize orprecipitate the borohydride compound product. The preferred method willdepend on the solubility-temperature profile of the solvent selected.Additional solvent washes can be used to improve recovery and purity.The metal oxide can be reduced back to the metal in a subsequent step todevelop a recycle loop for the process.

The liquid stream can also be withdrawn during the course of thereaction to remove the borohydride compound and the solvent returned tothe reactor to lower the reactor borohydride compound content and drivethe reaction further to completion. As such, reactions that may beequilibrium constrained may be enhanced for higher yields. The formationof borohydride is also highly exothermic. By cooling the solvent returnstream to the reactor, a means for controlling reaction temperature isalso provided. For example, the withdrawn solvent will be at the reactortemperature. If this stream is sufficiently cooled, borohydride compoundcrystals will form and can be removed using conventional methods asdescribed above. The cooled solvent of lower borohydride compoundcontent is returned to the reactor to maintain reactor temperature atthe target condition.

1. A process for production of a borohydride compound; said processcomprising steps of: (a) combining a boron-containing salt and siliconhydride to form a reaction mixture; and (b) grinding the reactionmixture formed in step (a) to form the borohydride compound.
 2. Theprocess of claim 1 further comprising combining a mineral oil with theboron-containing salt and silicon hydride to form the reaction mixture.3. The process of claim 1 in which the borohydride compound is sodiumborohydride.
 4. The process of claim 1 further comprising a catalystselected from the group consisting of powders of the transition metalsand their oxides.